SB Tactical posted that they heard back from the ATF and that there is a reversal ruling concerning arm braces. If you download the letter from the ATF, you will see at the bottom of the second page this paragraph:
“With respect to stabilizing braces, ATF has concluded that attaching the brace to a handgun as a forearm brace does not “make” a short-barreled rifle because in the configuration as submitted to and approved by FATD, it is not intended to be and cannot comfortably be fired from the shoulder. If, however, the shooter/possessor takes affirmative steps to configure the device for use as a shoulder-stock for example, configuring the brace so as to permanently affix it to the end of a buffer tube, (thereby creating a length that has no other purpose than to facilitate its use as a stock), removing the arm-strap, or otherwise undermining its ability to be used as a brace – and then in fact shoots the firearm from the shoulder using the accessory as a shoulder stock, that person has objectively “redesigned” the firearm for purposes of the NFA. This conclusion is not based upon the mere fact that the firearm was fired from the shoulder at some point. Therefore, an NFA firearm has not necessarily been made when the device is not re-configured for use as a shoulder stock – even if the attached firearm happens to be fired from the shoulder.”
Notice the bold? "not necessarily" may not mean what we want it to mean, it may mean it, who knows. So shoulder it, don't shoulder, even though something changed, things still appear to be the same. As always, use common sense.